Field
This invention relates generally to a system and method for detecting and tracking intense tornadoes and, more particularly, to a system and method for detecting and tracking intense tornadoes using an unmanned aerial vehicle (UAV) flying above a super-cell or mesocyclone, where the UAV includes sensors and detectors for detecting tornadogenesis by detecting formation of an eye-type structure near the center of the super-cell or mesocyclone.
Discussion
Certain areas of the country and worldwide are susceptible to the formation of tornadoes, especially at certain times of the year. For example, especially during the spring and fall on the Great Plains and in the Midwest of the United States, low-level, southerly, relatively warm and moist Gulf air that runs under mid- and upper-tropospheric, westerly, relatively cold, dry, fast-flowing air off of the Rocky Mountains often operates to create thunderstorms. If a thunderstorm transitions into a strong rotating vortex, termed a super-cell or mesocyclone, it could spawn one or more intense tornadoes. More specifically, most thunderstorms form in a tropospheric stratification characterized by convective available potential energy (CAPE) of 2000-3000 m2/s2. Further, a thunderstorm may develop in the presence of large vertical wind shear, such as >20 m/s below 4 km. Such wind shear then tilts a slow-to-develop convective cell perhaps 25° from vertical, so that a canted updraft rains out into a flanking downdraft. At the surface, the outflow from the downdraft lifts more convectively unstable air to its level of free convection so that an exceptionally long-lived, large-diameter thunderstorm persists. A subset of such thunderstorms can develop organized cyclonic rotation up to about a 25 m/s peak swirl, as indicated by a propensity to translate to the right of the pressure-weighted mean tropospheric wind. The fully developed rotating mesocyclone or super-cell may extend to a height of 15 km, and may persist in a nearly steady configuration for hours.
Depending on the characteristics of the parent thunderstorm, tornadoes of different sizes and intensities, generally classified on an extended Fujita (EF) scale from 0-5 on the basis of post-event damage assessment, can develop. A typical tornado is relatively small in diameter, is short-lived, and has a short path of narrow swath. A small percentage of tornadoes are intense tornadoes that are generally categorized in the EF 3-5 range and can be very destructive, often resulting in loss of life. Such long-path, long-lived, wide-swath, rapidly translating, high-swirl tornadoes are spawned virtually exclusively within super-cells. Early detection of very intense tornadoes to afford a long warning time is not only very important, but is relatively difficult.
The National Oceanic and Atmospheric Administration (NOAA) is a U.S. government agency that detects and tracks storms, and provides warnings to the public when appropriate. An important current basis for issuing a tornado warning relies on the use of Doppler radar, such as NEXRAD (next generation radar), which detects a rotating super-cell using the Doppler effect. NOAA's specific radar system for issuing tornado warnings employs dual-Doppler radars that are deployed on a roughly 240-km grid. These radar systems typically detect the parent mesovortex because they are rarely close enough to the super-cell to resolve an embedded tornado. Because the width of a tornado typically is less than the width of the spreading radar beam, Doppler radar provides, at best, a degraded indication of tornadogenesis in a mesocyclone, except for the rare case of a large tornado within a few km of the radar site. Thus, these types of radars are often not able to determine whether the center of a detected mesocyclone storm contains a tornado.
Only about 1 in 4 or 5 thunderstorms with organized rotation spawns even one long-path tornado, which is not inconsistent with NOAA's approximately 75% false-alarm rate, possibly resulting in warnings fatigue by residents of tornado-prone locales. NEXRAD radars are being upgraded with dual-polarization capability, but that is of limited effect for tracking long-lived rapidly translating super-cells with a possible embedded tornado. NOAA has also considered deploying short-range radars to fill gaps between the radar nodes in the NEXRAD network. However, this and other radar upgrades do not appear to be economically feasible or provide a reduced false-alarm rate of detecting tornadoes, especially intense tornadoes.
It has been postulated in the art that very intense tornadoes have significant differences from tornadoes of lesser intensity that may not be as destructive. Further, it has been proposed in the art that the occurrence of mid-latitude intense tornadoes involves transitions in the structure of the parent vortex that is analogous to the transition well known to occur in intense tropical cyclones. It has been recognized that in a tropical cyclone there are many different histories that may lead to attainment of the tropical-depression stage, but the varied antecedents make little difference once the tropical-depression stage is reached. Similarly, while non-axisym metric, slant-wise convection, and wind-veer/wind-shear may be essential to inception stages, once a mesocyclone is formed, the mid-latitude vortex is roughly axisymmetric, and the mid-latitude vortex thenceforth shares many features with its tropical counterpart, albeit on vastly reduced temporal and lateral spatial scales. It is noted that a mesocyclone can achieve and lose tornadic-stage intensity several times. Thus, a single mesocyclone may go through a succession of 45-minute-or-so tornadic stages, just as tropical cyclones can revert back and forth between being a tropical storm and a hurricane. A major hurricane is a hurricane with an eye that extends most of, if not all, the way from the tropopause to sea level.
Therefore, the process by which a tropical depression typically evolves to a tropical storm, and then sometimes to a hurricane (or typhoon) including a centrally sited eye, may entail a process similar to how a mid-latitude mesocyclone may intensify further and achieve an intense tornadic stage. Thus, in order to detect these high-intensity tornadoes, it may be desirable to detect the formation of an eye-type structure, identified here as tornadogenesis, and indicative of significant further intensification in a super-cell. Tornadogenesis is the super-cell transitioning from a large rotating thunderstorm to an appreciably more intense vortex with an altered core structure. This transition is sometimes referred to in the industry as a transition from a one-cell structure to a two-cell structure, where the one-cell structure is a rotating super-cell without an eye, and the two-cell structure is a rotating super-cell with an eye surrounded by an eyewall. As is known in the art, the eye of a hurricane has relatively low turbulence, and the eyewall, the part of the storm adjacent to the eye, has the most violent turbulence. The same properties plausibly hold for the evolving super-cell.